The investigator proposes to continue study of the pathophysiologic mechanisms of alterations in calcium, magnesium, and phosphate excretion. Microperfusion of isolated tubular segments from the kidneys of rabbits will be utilized to determine the site and mechanisms of increased calcium excretion in phosphate depletion. In addition factors which regulate urinary phosphate excretion such as PTH and pCO2 will be studied using this preparation to determine where in the kidney tubule these effects are exerted. Finally, studies will be carried out to investigate the control of the renal handling of magnesium in various tubular segments and the relationship of magnesium transport to that of calcium and phosphate.